1. Field of the Invention
The subject invention relates to heat-curable epoxy resin systems having exceptional resistance to impact induced damage. More particularly, the subject invention relates to toughened epoxy resin systems which when cured are monophasic and which are capable of preparing carbon reinforced composites having compression after impact values greater than 40 Ksi.
2. Description of the Related Art
Epoxy resins have been widely used in the field of structural materials as matrix resins for fiber reinforced prepregs and as structural adhesives. However, it is difficult to achieve both high strength and toughness in epoxy systems, even when cured with curing agents such as the diaminodiphenylsulfones. When used as prepregging resins with carbon fibers, for example, such systems typically have compression after impact (CAI) values of only about 20-30 Ksi after an impact of 1500 in-lb/in.
In the past, high CAI values have required resort to multiphase systems. For example, in EP-A-0 252 725, epoxy resins are combined with epoxy or amine functional polyarylene ether sulfone oligomers, epoxy curing agents, and from 1 to about 16 percent of an infusible elastomer having a particle size of from 10 to about 75 .mu.m. In addition to the discontinuous phase created by the infusible rubber particles, the cured epoxy system itself is multiphasic, having a discontinuous glassy epoxy phase dispersed in a continuous glassy polysulfone thermoplastic rich phase.
In EP-A-0 193 082 (U.S. Pat. Nos. 4,656,207 and 4,656,208) are disclosed multiphasic systems similar to those of EP-A-O 252 725 except that the infusible particles are omitted. Both of these resin systems are said to allow preparation of carbon fiber reinforced composites having CAI values of from 40-50 Ksi after having been subjected to an impact of 1500 in-lb/in.
However, the morphology of these and other multiphasic systems is cure cycle dependent and thus small variations in the cure conditions can alter the morphology of these systems in an unpredictable manner. Furthermore, the presence of appreciable amounts of acrylonitrile/butadiene or other common elastomers generally causes both a decrease in glass transition temperature and a loss in high temperature strength.
Thus, it would be highly desirable to formulate a resin system which cures to form a single phase system, but which may be used to prepare carbon composites which exhibit CAI values greater than 40 Ksi after a 1500 in-lb/in impact.